What is duplex stainless steel pipe?

What is duplex stainless steel pipe?

What is duplex stainless steel pipe?

Compared with ferritic stainless steel and austenitic stainless steel, duplex stainless steel pipe has good welding performance. It is neither like the welding heat affected zone of ferritic stainless steel, because of the grain coarsening, the plastic toughness is greatly reduced, nor like austenitic stainless steel, it is more sensitive to welding hot cracks.

Due to its special advantages, duplex stainless steel is widely used in petrochemical equipment, seawater and wastewater treatment equipment, oil and gas pipelines, paper-making machinery and other industrial fields. In recent years, it has also been used in the field of bridge bearing structure, with a good development prospect.

In duplex stainless steel, ferrite and austenite account for half of each phase, and the content of the least phase may reach 30%.

Common grades

S31803/S32205, S31500, S32304, S32750, S32760

Comparison of the approximate values of the commonly used two-phase stainless steel grades and the grades of various countries is shown in the table below:

Model / Country	China	U.S.A	Sweden	Germany	France	Japan
Low alloy type	00Cr23Ni4N	UN23 (SAF2304）	SS232 （SAF2304）	W．Nr.1.4362	UR35N	DP11
Medium alloy type	00Cr18Ni5Mo3Si2 00Cr22Ni5Mo3N	UNS S31500 UNS S31803	SS2376(3RE60) SS2377(SAF2205)	W.Nr.1.4417 W.Nr.1.4462	UR45N	DP1 DP8
High alloy type	0Cr25Ni5Mo2 00Cr25Ni7Mo3WCuN	UNS S32900 UNS S31260	SS2324(10RE51)	W.Nr.1.4460 W.Nr.1.4501		329J1 329J2L
Super duplex steel	00Cr25Ni7Mo4N 00Cr25Ni6Mo3CuN	UNS S32750 UNS S32550	SS2328(SAF2507)	W.Nr.1.4410 W.Nr.1.4507		UR47N+ UR52N+

Common standards

• ASTM A789 (ASME sa789) seamless and Welded Ferritic / austenitic stainless steel pipe for general purpose
• ASTM A790 (ASME sa790) seamless and Welded Ferritic / austenitic stainless steel nominal pipe
• GB / T 21833-2008 AUSTENITIC FERRITIC duplex stainless steel seamless pipe

Properties of commonly used duplex stainless steel:

Chemical composition (%)

Grades	C≤	Mn≤	Si≤	S≤	P≤	Cr	Ni	Mo	Cu≤	N
S32750((SAF2507) 00Cr22Ni7Mo4N	0.03	1.20	0.80	0.020	0.035	24.0/ 26.0	6.0/ 8.0	3.0/ 5.0	0.50	0.24/ 0.32
S31803（SAF2205） 00Cr22Ni5Mo3N	0.03	2.00	1.0	0.02	0.030	21.0/ 23.0	4.50/ 6.50	2.50/ 3.50		0.08/ 0.20
S31500（3RE60） 00Cr18Ni5Mo3Si2	0.03	1.2/ 2.00	1. 4/ 2.00	0.030	0.030	18.0/ 19.0	4.25/ 5.25	2.50/ 2.00		0.05/ 0.10

Mechanical properties:

	Heat treatment temperature/℃	Ab（MPa）≥	As（MPa）≥	∮≥	Brudzinski	Rockwell
S32750（SAF2507） 00Cr22Ni5Mo3N	1025-1125	800	550	15	310	32
S31803（SAF2205） 00Cr22Ni5Mo3N	1020-1100	620	450	25	290	30.5
S31500（3RE60） 00Cr18Ni5Mo3Si2	980-1040	630	440	30	290	30.5

The dual phase steel / super duplex steel stainless steel pipe is a kind of stainless steel with high strength and high corrosion resistance, which has the properties of austenitic stainless steel and ferritic stainless steel. Generally, when the carbon content is low, the Cr content is 18% ~ 28%, and the Ni content is 3% ~ 10%. Some steels also contain Mo, Cu, Nb, Ti, N and other alloy elements.
Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance, and maintained 475 ℃ brittleness, high thermal conductivity and Superplasticity of ferritic stainless steel. Compared with austenitic stainless steel, its yield strength is significantly improved, and its resistance to intergranular corrosion, stress corrosion, corrosion fatigue and abrasion is significantly improved.

Application of duplex stainless steel pipe

Chemical and petrochemical processing industry

This field involves a wide range, complex working conditions and various media. It is also the earlier and more fields where duplex stainless steel is used. At present, the catalyst tubes of large-scale and multiple sets of small and medium-sized methanol synthesis reactors with a capacity of less than 200000 tons are made of duplex stainless steel, 2205 steel tubes are used in large and medium-sized units, many imported tubes are used, and 18-5Mo domestic steel tubes are used in small-scale units. The medium condition (HCI, water vapor) of the cooling snake tube in the oxychlorination reactor of the vinyl chloride unit of Qilu Petrochemical Company is very strict. At present, the imported 2205 duplex stainless steel tube has been used, and the use result remains to be observed. The catalyst regeneration cooler of ethylene plant is made of 00c25ni7mo3wcun duplex stainless steel similar to DP3 steel. The outlet temperature of seawater is 40t, which has been used intermittently for 15 years. The effect is very good. Several coolers of the pulverized coal gasification plant in Henan coal chemical plant are made of imported 2205 steel pipes. In the next five years, China’s petrochemical industry will develop rapidly, with large-scale petrochemical projects under construction and to be built all over the country. According to the current projects under construction and planned construction, the annual demand for stainless steel pipes in the petrochemical industry in 2005 is about 16000 tons. With the improvement of the quality of domestic stainless steel products and the development of special brand products, the application of stainless steel in China’s petrochemical industry will continue to increase in the next five years.

Oil and gas industry

This is one of the main fields of the application of duplex stainless steel in foreign countries. At present, 1000 km of oil and gas transmission pipeline has been laid. Only a small amount of Nanhai oil field is used in China, all of which are imported. In addition, dual phase stainless steel welded pipe is considered for the gas gathering pipeline from Tarim Basin in the west of West to east gas transmission project, which has been produced and manufactured in China. The oil refining industry is the first department to use domestic duplex stainless steel. In Nanjing, Zhenhai, Tianjin, Jinan and other refining and chemical companies, it is mainly used in the overhead lining (or composite plate), tower internals, air coolers and water coolers of atmospheric and vacuum distillation tower. The longest one has been used for 20 years. For the production of small size duplex stainless steel seamless pipe, it has been relatively mature in China. At present, the production of large size seamless pipe and welded pipe is in urgent need of solution, and the production process needs to be developed. In addition, how to reduce the price of duplex stainless steel products is also related to the development of domestic duplex stainless steel. Although it is very important to expand the output and produce marketable products, it can not be ignored that we should adopt new technology and new process, adopt molybdenum oxide ball instead of metal molybdenum, nickel oxide instead of metal nickel, reduce the smelting cost, promote the adoption of duplex stainless steel continuous casting technology, and improve the yield.

Application in fertilizer industry

The fertilizer industry adopts the domestic 0cr25ni6mo3cun aging strengthening duplex stainless steel, which is used for the internals of the high-pressure stop valve of various specifications in the main process pipeline of the urea plant with its wear resistance and corrosion resistance. The effect is good. At present, the CO2 stripping method and NH3 stripping method for urea production mostly adopt high alloy austenitic stainless steel, even titanium material. In China, high chromium duplex stainless steel has been used to replace 316L (urea grade) steel on the pump body of carbamate pump in large-scale urea plant. Since the 1980s, dp-12 (25cr-7ni-3mo-n Steel Containing Trace Tungsten and copper) and R-5 (00cr25ni6.5mo1.5n) duplex stainless steel have been used in the three high-pressure equipment of aces urea new process developed by Toyo Engineering Company of Japan, the former is used for manufacturing stripping pipe, condenser pipe, decomposition pipe and transmission pipe, and the latter is used for shell of high-pressure equipment 。 In 2005, the annual demand for stainless steel pipe in petrochemical industry was about 7800t, which is expected to increase to 9600t in 2010. The national standard of stainless steel has not been revised for more than ten years. The acceptance and revision of duplex stainless steel must be considered when revising the national standard. In order to promote the production and ordering of duplex stainless steel, we should also consider the introduction and absorption of the existing inspection methods and standards. In addition, we should vigorously explore the potential market for industrial and civil use of duplex stainless steel (such as construction industry – structural, bridge, concrete reinforcement; manufacturing industry – container, storage tank, chemical transport ship, speedboat, tank car; and household facilities – high water tank, electric water heater, residential water and auto pipe, etc.), and develop, test and promote the products developed abroad that are suitable for China’s national conditions (less nickel) Some new varieties of chromium (such as low nickel and energy-saving duplex stainless steel) are believed to have a very broad application prospect in promoting duplex stainless steel in China.

Restrictions and requirements for the safe use of duplex stainless steel pipes

• 1. It is necessary to control the phase proportion. The most appropriate proportion is that the ferrite phase and austenite phase account for about half each, and the number of a certain phase cannot exceed 65% at most, so as to ensure the best comprehensive performance. If the ratio of two phases is out of balance, for example, too many ferrite phases, it is easy to form single-phase ferrite in welding HAZ, which is sensitive to stress corrosion cracking in some media.
• 2. It is necessary to master the rule of microstructure transformation of duplex stainless steel pipe, and be familiar with TTT and CCT transformation curve of each steel type. This is the key to correctly guide the formulation of heat treatment, thermoforming and other processes of duplex stainless steel. The precipitation of brittle phase of duplex stainless steel is more sensitive than that of austenitic stainless steel.
• 3. The continuous service temperature range of duplex stainless steel pipe is – 50 ~ 250 ℃, the lower limit depends on the brittleness transition temperature of steel, the upper limit is limited by 475 ℃ brittleness, and the upper limit temperature cannot exceed 300 ℃.
• 4. After solution treatment, dual phase stainless steel pipes need fast cooling. Slow cooling will cause the precipitation of brittle phase, which will lead to the decrease of toughness, especially local corrosion resistance of steel.
• 5. The lower limit temperature of hot working and hot forming of high chromium molybdenum duplex stainless steel tube shall not be lower than 950 ℃, and that of super duplex stainless steel shall not be lower than 980 ℃, and that of low chromium molybdenum duplex stainless steel shall not be lower than 900 ℃, so as to avoid surface cracks caused by the precipitation of brittle phase in the processing
• 6. The stress relief treatment of 650-800 ℃ commonly used for austenitic stainless steel cannot be used, and the solution annealing treatment is generally used. After surfacing of duplex stainless steel on the surface of low alloy steel, the toughness and corrosion resistance caused by the precipitation of brittle phase, especially the decrease of local corrosion resistance, must be taken into account when the overall stress relief treatment at 600-650 ℃ is needed, so as to shorten the heating time in this temperature range as much as possible. The heat treatment of low alloy steel and duplex stainless steel composite plates should also be considered.
• 7. It is necessary to be familiar with the welding rules of duplex stainless steel pipes, not all of which can be welded with austenitic stainless steel. Whether the equipment of duplex stainless steel can be used safely or not is closely related to the correct control of the welding process of steel. The failure of some equipment is often related to welding. The key lies in the control of line energy and interpass temperature. It is also very important to select welding materials correctly. The two-phase ratio of the welded joint (weld metal and welding HAZ), especially the amount of austenite in the welding HAZ, is very important to ensure that the welded joint has the same performance as the base metal.
• 8. When choosing duplex stainless steel in different corrosion environment, it should be noted that the corrosion resistance of steel is always relative. Although duplex stainless steel has better local corrosion resistance, for a certain duplex stainless steel, it also has a suitable range of medium conditions, including temperature, pressure, medium concentration, pH value, etc., which should be carefully selected. Many of the data obtained from the literature and manual are laboratory corrosion test results, which are often different from the actual conditions of the project. Therefore, attention should be paid to the selection of materials. If necessary, corrosion test in the actual medium or coupon test under the field conditions, or even the test of simulation device should be carried out.

Characteristics of duplex stainless steel

Due to the characteristics of two-phase structure, DSS has the advantages of both ferritic stainless steel and austenitic stainless steel through the correct control of chemical composition and heat treatment process.

Advantages of duplex stainless steel compared with austenitic stainless steel

• (1) The yield strength of austenitic stainless steel is more than twice that of ordinary stainless steel, and it has enough plastic toughness for forming. The wall thickness of the tank or pressure vessel made of duplex stainless steel is 30-50% less than that of austenite, which is helpful to reduce the cost.
• (2) Even duplex stainless steel with the lowest alloy content has a higher ability of stress corrosion cracking resistance than austenitic stainless steel, especially in chloride containing environment. Stress corrosion is an outstanding problem that is difficult to solve for ordinary austenitic stainless steel.
• (3) The corrosion resistance of 2205 duplex stainless steel, which is widely used in many media, is better than that of 316L austenitic stainless steel, while super duplex stainless steel has very high corrosion resistance. In some media, such as acetic acid, formic acid, etc., it can even replace high alloy austenitic stainless steel and even corrosion resistant alloy.
• (4) Compared with austenitic stainless steel with the same alloy content, it has better wear resistance and fatigue corrosion resistance than austenitic stainless steel.
• (5) Compared with austenitic stainless steel, the coefficient of linear expansion is lower, close to carbon steel, suitable for connection with carbon steel, which has important engineering significance, such as production of composite plates or linings.
• (6) No matter in the condition of dynamic or static load, it has higher energy absorption capacity than austenitic stainless steel, which has obvious advantages and practical application value for structural members to deal with sudden accidents such as collision, explosion, etc.

The disadvantage of duplex stainless steel compared with austenitic stainless steel

• (1) The universality and versatility of application are not as good as austenitic stainless steel, for example, its service temperature must be controlled below 250 ℃.
• (2) Its plastic toughness is lower than that of austenitic stainless steel, and its cold, hot working process and forming properties are not as good as austenitic stainless steel.
• (3) In order to avoid the appearance of harmful phase and damage the performance, the process system of heat treatment and welding should be strictly controlled.

Advantages of duplex stainless steel compared with ferritic stainless steel

• (1) The comprehensive mechanical properties of ferritic stainless steel are better than that of ferritic stainless steel, especially the plastic toughness, which is not as sensitive to brittleness as ferritic stainless steel.
• (2) In addition to stress corrosion resistance, other local corrosion resistance are better than ferritic stainless steel.
• (3) The properties of cold working process and cold forming are much better than that of ferritic stainless steel.
• (4) The welding performance is also far better than that of ferritic stainless steel. Generally, preheating is not required before welding and heat treatment is not required after welding.
• (5) The application range is wider than that of ferritic stainless steel.

The disadvantage of duplex stainless steel compared with ferritic stainless steel

The alloy element content is high, the price is relatively high, the general ferrite does not contain nickel.

Advantage comparison between 2205 duplex stainless steel tube and 316L stainless steel tube

Compared with austenitic 316L steel pipe, 2205 duplex stainless steel pipe has the following advantages:

• 1. The yield strength of 2205 duplex steel is more than twice that of 316L austenitic stainless steel, and it has enough plastic toughness for forming. The wall thickness of pressure vessel made of 2205 stainless steel tube is 30-50% less than that of 316L stainless steel tube, which is beneficial to reduce the cost.
• 2. 2205 steel pipe has excellent resistance to stress corrosion cracking, especially in the marine environment containing chloride ions, stress corrosion is a serious problem for austenitic stainless steel.
• 3. In many media, the corrosion resistance of 2205 duplex steel is due to the common 316L stainless steel, while the super duplex stainless steel has very high corrosion resistance. For example, in acetic acid, formic acid and so on, it can even replace the high alloy austenitic stainless steel pipe, even the corrosion-resistant alloy.
• 4. Compared with austenitic stainless steel with the same alloy content, 2205 duplex stainless steel has good local corrosion resistance. In view of its high strength and good corrosion resistance, its wear resistance and corrosion fatigue performance are due to the general 316L austenitic stainless steel.
• 5. The linear expansion coefficient of 2205 duplex steel is lower than that of austenitic stainless steel, which is close to that of carbon steel.

Compared with 316L stainless steel pipe, 2205 stainless steel pipe has the following disadvantages:

• 1. The application of 2205 dual phase steel is not as universal and multifaceted as 316L austenitic stainless steel. For example, the medium temperature must be controlled below 250 ℃ for long-term application.
• 2. Compared with 316L stainless steel, 2205 dual phase steel has lower plastic toughness, worse cold and hot working process and formability.
• 3. 2205 dual phase steel has medium temperature brittleness, so it is necessary to strictly control the process system of heat treatment and welding in order to avoid the occurrence of harmful phases and damage the properties.

Source: China Duplex Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Reference:

• https://www.steeljrv.com
• https://www.yaang.com